Vinurinn fer inn í húsið.

In Icelandic, definiteness is marked by a suffix on the noun rather than a separate article.

• Vinur means “a friend.”
• Adding -inn (the masculine singular definite ending) gives vinurinn, “the friend.”

Hús is a neuter noun. The definite singular ending for neuter nouns in both nominative and accusative is -ið.

• hús = “a house” (indefinite)
• húsið = “the house” (definite)

They work together to express movement into an enclosed space:

• inn is an adverb or directional particle meaning “in(wards).”
• í is the preposition “into.”
  Combined, inn í means “into” with emphasis on going inside.
  If you wanted to say “is inside,” you’d use inni í (stative) instead of inn í (dynamic):
  “Vinurinn er inni í húsið.” (“The friend is inside the house.”)

Prepositions in Icelandic trigger different cases depending on meaning.

• í
  • movement (direction) → accusative
• í
  • location (static) → dative
    Since inn í indicates “going into,” húsið is accusative. For neuter nouns the accusative form looks identical to the nominative: húsið.

Fer is the 3rd person singular present-tense form of the irregular verb fara (“to go”). Icelandic verbs change form for person and number:

• ég fer (I go)
• þú ferð (you go)
• hann/hún fer (he/she goes)
• við förum (we go), etc.

Icelandic generally follows the V2 rule (verb-second):
1) Some element (often the subject or an adverb) comes first.
2) The finite verb is second.
3) The rest of the clause follows.
In Vinurinn fer inn í húsið:

• Vinurinn (subject) is first,
• fer (finite verb) is second,
• inn í húsið completes the sentence.

Adverbs can go:

• Immediately after the verb: Vinurinn fer hljóðlega inn í húsið.
• At the very end: Vinurinn fer inn í húsið hljóðlega.
  Both are grammatically correct; the first is more neutral, the second can slightly emphasize how he enters.